In optical communications networks, optical transceiver modules are used to transmit and receive optical signals over optical fibers. On the transmit side of a transceiver module, a light source (e.g., a laser diode) generates amplitude modulated optical signals that represent data, which are received by an optics system of the transceiver module and focused by the optics system into an end of a transmit optical fiber. The signals are then transmitted over the transmit fiber to a receiver node of the network. On the receive side of the transceiver module, the optics system of the transceiver module receives optical signals output from an end of a receive optical fiber and focuses the optical signals onto an optical detector (e.g., a photodiode), which converts the optical signals into electrical signals. The electrical signals are then processed to recover the data contained in the electrical signals.
The transmit and receive fiber cables have connectors on their ends (e.g., LC connectors), that are adapted to mate with transmit and receive receptacles, respectively, formed in the transceiver module. A variety of optical transceiver module configurations are used in optical communications network. Some optical transceiver modules have multiple laser diodes on the transmit side and multiple photodiodes on the receive side for simultaneously transmitting multiple optical signals and receiving multiple optical signals, respectively. In these types of transceiver modules, the transmit fiber cables and the receive fiber cables have multiple transmit and multiple receive optical fibers, respectively. The transmit and receive fiber cables are typically ribbon cables having ends that are terminated in a connector module that is adapted to be plugged into a receptacle of the transceiver module.
Some optical transceiver modules have a single laser diode on the transmit side and a single photodiode on the receive side for simultaneously transmitting an optical signal and receiving an optical signal over transmit and receive fiber cables, respectively. Each of the cables has a single transmit and a single receive fiber, respectively. The ends of the transmit and receive cables have connectors on them that are adapted to plug into transmit and receive receptacles, respectively, formed in the transceiver module. These types of transceiver modules are often referred to as pluggable transceiver modules. Small form-factor pluggable (SFP) and SFP+ transceiver modules are examples of pluggable optical transceiver modules.
Typically, pluggable transceiver modules, such as the SFP and SFP+ transceiver modules, for example, are designed to be inserted into cages. The pluggable transceiver modules and the cages have locking features disposed on them that allow the transceiver modules to mate with an interlock with the cages. The external surfaces of the cages are typically designed to be inserted into openings formed in racks, with each rack typically including many such openings for receiving many such cages. The pluggable transceiver modules typically include latch lock pins that are designed to be received in latch openings formed in the cages. In order to mate the pluggable transceiver module with the cage, the module is inserted into the cage and a latching mechanism is moved to a latching position to cause the latch lock pin on the transceiver module to be extended into the latch opening formed in the cage. In order to remove the transceiver module from the cage, the latching mechanism is moved to a de-latching position to cause the latch lock pin to be retracted from the latch opening, allowing the transceiver module to be pulled out of the cage.
In most pluggable optical transceiver module designs, the area around the latch lock pin constitutes an EMI open aperture that allows EMI to escape from the transceiver module housing. The Federal Communications Commission (FCC) has set standards that limit the amount of electromagnetic radiation that may emanate from unintended sources. For this and other reasons, a variety of techniques and designs are used to shield EMI openings in transceiver module housings in order to limit the amount of EMI that passes through openings. For example, various metal shielding designs and resins that contain metallic material have been used to cover areas from which EMI may escape from the housings. So far, such techniques and designs have had only limited success, especially with respect to transceiver modules that transmit and receive data at very high data rates (e.g., 10 gigabits per second (Gbits/sec)).
EMI collars are often used with pluggable transceiver modules to provide EMI shielding. The EMI collars in use today vary in construction, but generally include a band portion that is secured about the exterior of the transceiver module housing and spring fingers having proximal ends that attach to the band portion and distal ends that extend away from the proximal ends. The spring fingers are periodically spaced about the collar. The spring fingers have folds in them near their distal ends that cause the distal ends to be directed inwards toward the transceiver module housing and to come into electrically conductive contact with the housing at periodically-spaced points on the housing. At the locations where the folds occur near the distal ends of the spring fingers, the outer surfaces of the spring fingers are in electrically conductive contact with the inner surface of the cage at periodically spaced contact points along the inner surface of the cage.
The amount of EMI that passes through an EMI shielding device is proportional to the largest dimension of an EMI open aperture in the EMI shielding device. In the known EMI collars used with optical transceiver modules, the spacing between the locations at which the distal ends of the spring fingers come into electrically conductive contact with the inner surface of the cage corresponds to the largest EMI open aperture dimension. The spacing between these contact points is generally equal to the spacing between the spring fingers. Consequently, the spacing between the spring fingers typically constitutes the largest EMI open aperture dimension on the collar. In general, the maximum EMI open aperture dimension needs to be no greater than one quarter of a wavelength of the frequency that is being attenuated. Even greater attenuation of the frequency of interest can be obtained by making the maximum EMI open aperture dimension significantly less than one quarter of a wavelength, such as, for example, one eighth or one tenth of a wavelength. However, the ability to decrease this spacing using currently available manufacturing techniques is limited. In addition, as the frequency or bit rate of the transceiver module increases, this spacing needs to be made smaller in order to effectively shield EMI. As the bit rates of these transceiver modules continue to increase, decreasing the spacing between spring fingers in order to improve EMI shielding effectiveness becomes less feasible, or impossible.
The EMI collars are typically secured to the transceiver module housings by placing the band of the collar in a recess formed in the housing. An adhesive material is contained in the recess that adheres the collar to the transceiver module housing. One of the problems associated with this method of attaching the collar is that it is susceptible to being damaged when the transceiver module housing having the collar secured thereto is inserted into a cage. During insertion of the module housing into the cage, the inner surface of the cage comes into contact with the housing and with the collar, causing forces to be exerted by the inner surface of the cage on the band and spring fingers of the collar. The exertion of such forces by the cage on the collar oftentimes deforms the collar, which can result in: (1) elimination of one or more of the electrically conductive contact points where the band of the collar and the module housing come into electrically conductive contact with each other; (2) elimination of one or more of the electrically conductive contact points where the distal ends of the spring fingers come into electrically conductive contact with electrically conductive contact points on the module housing; and (3) elimination of one or more of the electrically conductive contact points where the folds near the distal ends of the spring fingers come into electrically conductive contact with and the inner surface of the cage. The elimination of any of these electrically conductive contact points results in a reduction in EMI shielding effectiveness, which degrades performance.
Accordingly, a need exists for an EMI collar for use with a pluggable optical transceiver module that has improved EMI shielding over that provided by existing EMI collars used with pluggable optical transceiver modules.